This invention relates to a target assembly for use in a magnetron-type sputtering device.
In a sputtering device of the type described, a target assembly is located in a hollow space. The target assembly is for use in sputtering a target material onto at least one object by the help of an electric field and a magnetic field which intersects the electric field. Such a target assembly comprises a target member of the target material and a magnet member for producing the magnetic field in the hollow space. The electric field is produced in the hollow space between the target member and the at least one object.
The sputtering device is often used in fabricating an integrated circuit in forming a thin film of, for example, metal on an object, such as a semiconductor piece or wafer. It is to be noted here that the object is microscopically never uniform on a surface thereof, even though it may macroscopically appear uniform. In other words, the surface may approximately be considered as a combination of a planar portion and step portions or projections which are offset from the planar portion and which are contiguous to the planar portion through corners or edges of the step portions. On depositing the thin film, it is preferable that the thin film is substantially uniform on both of the planar portion and the step portions. Otherwise, discontinuity of the films takes place at the step portions.
As will later be described in conjunction with a few figures of the accompanying drawing, the thin film is apt to become thin on the corners of the step portions. Moreover, the thin film is not uniform even on the corner of a sole one of the projections because the corner of the sole projection is not always covered with the thin film on the same sputtering conditions. For example, the corner has an inner part near to a center of the object and an outer part farther therefrom. When both surfaces of the object and target are set in parallel and both center axes are roughly aligned with each other, a sputtered deposit usually becomes thin at the outer part in comparison with the inner part, as being described later.
Let a thickness of the thin film be measured in a direction perpendicular to the thin film. In this event, it is possible to define a ratio of the thickness which are measured on the corner of the sole projection and the planar portion. The ratio may be called a step coverage ratio. It may be said that the thin film is more excellent as the step coverage ratio becomes near to unity. Preferably, the step coverage ratio defined at the outer part of the corner should be equal to that defined at the inner part thereof.
As will later be described with reference to several of about ten figures of the accompanying drawing, a conventional target assembly is defective in that the step coverage ratio becomes distant from unity as the projections are spaced away from the center of the object. Furthermore, the step coverage ratio is inevitably varied even at the inner and the opposite parts of the sole projection from each other. As a result, the conventional target assembly is incapable of forming a uniform film onto the nonuniform surface of the object.